Dimensional reduction has been ongoing in a semiconductor process. Unlike a digital circuit block, however, such dimensional reduction in a process does not result in reduction in area of an analog circuit block. Moreover, since an LSI (Large Scale Integrated circuit) has multiple functions, power consumption has been increasing and reduction in power consumption has been a big challenge.
Conventionally, a current steering DAC has a plurality of unit current sources corresponding to the number of bits of a digital input signal, and a bias circuit for generating a bias voltage for adjusting current values of the unit current sources to a prescribed value. When a digital input signal is applied to the current steering DAC, the current steering DAC selects currents to be applied from the plurality of unit current sources to an analog output terminal according to the digital input signal. The current steering DAC then sums the selected currents and outputs the sum from the analog output terminal.
A multi-channel current steering DAC is a current steering DAC for multiple use having a plurality of channels of the same structure. In many of such multi-channel current steering DACs, the bias circuit is shared by the plurality of channels in order to reduce the area.
FIG. 8 shows an example of the structure of a current steering DAC. In FIG. 8, I1, I2, . . . indicate a plurality of current sources, DS indicates a digital input signal, 1 indicates a bias circuit, 3 indicates an analog output terminal, 4 indicates a stabilizing capacitor, and SW1, SW2, . . . indicate switch circuits. Each switch circuit SW1, SW2, . . . switches a current of a corresponding current source I1, I2, . . . between an analog output terminal 3 and the ground according to the value of a digital input signal DS.
This current steering DAC includes a plurality of current sources I1, I2, . . . corresponding to the number of bits of the digital input signal DS. For example, a thermometer-type current steering DAC includes 256 current sources in the case of an 8-bit digital input signal DS and includes 1,024 current sources in the case of a 10-bit digital input signal DS. A binary-type current steering DAC includes eight weighted current sources in the case of an 8-bit digital input signal DS. The switch circuits SW1, . . . are switched according to the value of the digital input signal DS so that a current corresponding to the value of the digital input signal DS flows into the analog output terminal 3 and an analog output is obtained. A stabilizing capacitor 4 for reducing an impact of noise such as crosstalk is connected in an output path of a bias voltage from a bias circuit 1.
The switch circuits SW1, . . . are usually formed by transistors. FIG. 9 shows an example of the switch circuit SW1. In FIG. 9, the digital input signal DS is decoded by a decoder 10 and the decoded digital signal is applied to the gate of one of two Pch transistors P1 and P2 of the switch circuit SW1, that is, to the gate of the Pch transistor P1. The decoded digital signal is applied to the gate of the other Pch transistor P2 through an inverter. In the case where a corresponding value of the decoded digital signal is at “L” level, the Pch transistor P1 is turned on and connects a current of the current source I1 to the analog output terminal 3. In the case where a corresponding value of the inverted decoded digital signal is at “H” level, the other Pch transistor P2 is turned on and a current of the current source I1 is connected to the ground.
FIG. 10 shows the structure of a multi-channel current steering DAC. In the example of FIG. 10, the current steering DAC has two channels A, B. A bias voltage output from the bias circuit 1 is shared for current adjustment of each current source in the two channels A, B.
Conventionally, an example of a method for changing a full-scale current in such a multi-channel current steering DAC is to switch the bias voltage of the bias circuit. A full-scale current is the sum of currents flowing from all the current sources to the analog output terminal. For example, in Patent document 1, a full-scale current is changed by changing a current value of each current source by switching a bias voltage generated by a bias circuit.
Patent document 1: Japanese Laid-Open Patent Publication No. 8-274642 (pp. 3-4, FIG. 1)